This page explains most of it: http://www.trulia.com/submit_listings/

Listings are either: individually entered by agents, fed to Trulia in an XML format by real estate firms (or other aggregators) or screen scraped by Trulia. They also I think have deals with some MLSs to bulk send them listings for member firms.

I work for http://www.streeteasy.com we do this for NYC.

Off topic, but Street Easy kicks serious butt. If anyone living in NYC is ever looking for a place to buy/rent, start there. I don't work for the company, I'm just an avid fan and frequent user. And I recommend it to everyone I know.

Thanks, we work hard at it :)